1. Field of the Invention
This invention relates primarily to the stitch-forming section of a sewing machine in which a bobbin is installed in a stationary bobbin case inside a rotary hook which is mounted on a horizontal shaft and rotates in a vertical plane.
2. Description of the Prior Art.
In prior art, in a sewing machine of the rotary hook and stationary bobbin type, a bobbin is usually mounted on a center sleeve or tube in a bobbin case cap which is installed over a center post in a bobbin case base rotatably mounted and retained in the rotary hook. A bobbin tension device is usually mounted on the exterior of the closed end of the bobbin case cap and a latch on the cap secures the cap to the end of the center post. The bobbin case cap has a bobbin extractor mechanism which, when the bobbin case cap is unlatched, grasps a flange of the bobbin. Removal of the bobbin case cap also removes the bobbin from the bobbin case base. To remove and replace the bobbin, the operator must remove the bobbin case cap which is subject to loss and damage. When installed, the bobbin and thread thereon are obscured by the bobbin case cap and the operator cannot ascertain the amount of thread on the bobbin. PRESTON, U.S. Pat. No. 3,476,068, eliminates the bobbin case cap; a latch is mounted on the center post of his bobbin case and a separate bobbin ejector mechanism is mounted on the sidewall of the bobbin case; means for applying tension to the bobbin thread applies a drag or braking force directly to a flange of the bobbin. The tension applied is described as independent of the type or thickness of the bobbin thread, but the tension must vary inversely as the amount of thread on the bobbin if the friction applied to the bobbin is constant. Transparent bobbins and bobbins with cutouts are known, as are viewing windows.